Geoffrey W. Stevens

Climate change mitigation strategies have driven research on energy-efficient sorbents for direct air capture (DAC). Ionic liquids (ILs) offer a flexible design and the possibility of integration with dielectric heating, presenting an opportunity to enhance cyclic CO2 recovery and significantly reduce the cost and energy consumption in DAC processes. Building upon an experimentally validated strategy to synthesize a range of quaternary ammonium salts with amino acid anions and a quat molecule counterion, a sustainable IL, [N4444][Tau+Lys], was designed to be coupled with microwave irradiation. This integrated approach led to a substantial enhancement in regeneration performance compared with conventional thermal heating, increasing the maximum CO2 desorption rate and total recovered CO2 by up to ∼496% and ∼66%, respectively. A comparative analysis of energy consumption further demonstrated an ∼62% reduction in total input energy when using microwave heating (0.26 kWh versus 0.69 kWh). The mechanism through which microwave radiation assists in overcoming energy barriers, thereby enhancing the carbamate decomposition reaction rate, has been investigated. It is recognized that the low ionization energy of amino sulfonic acid (taurine) combined with the high proton affinity of lysine in [N4444][Tau+Lys] not only reduces the viscosity of IL and influences the formation of polar dianion intermediates but also facilitates the crossing of the proton transfer energy barrier and coupling persistent transition states with microwave irradiation in each energy cycle.

Direct Air Capture is a promising climate mitigation technology, but its deployment is limited by high energy demand. This study improves the energy efficiency and sustainability of liquid-based Direct Air Capture by integrating catalytic solvent regeneration and hybrid solvents with a low-temperature membrane vacuum regeneration process. Iron-sulfated zirconia catalysts supported on alumina and silica are synthesized and evaluated, with silica exhibiting superior catalytic performance. An optimal iron-sulfated zirconia to silica ratio of 1:1 reduces relative heat duty by up to 59.7% in the membrane vacuum system. To assess the impact of hybrid solvents on energy consumption, a solvent composed of 3 molar potassium taurinate and 1 molar potassium sarcosinate achieves a 69.1% improvement in CO₂ desorption compared to potassium taurinate alone. Finally, combining the optimized hybrid solvent, catalyst, and membrane system at 90 °C reduces thermal energy consumption by 66.8% relative to potassium glycinate, achieving an energy requirement of 2.6 GJ/tCO₂.